The present invention relates to a method of producing a polycrystalline semiconductor thin film such as a polycrystalline silicon thin film used as a raw semiconductor material for a semiconductor element or a wiring material.
For example, a low pressure CVD method, that is, a chemical vapor deposition method under a low pressure is known as a method of producing a polycrystalline silicon thin film. FIG. 2 is a conceptional view showing the low pressure CVD method. In the drawing, a quartz boat 23 having single crystal silicon substrates 11 vertically set thereon is disposed in a transparent quartz pipe 22 which is disposed in a tubular electric furnace 21, and the pipe is exhausted by a vacuum exhaustion pump through an exhaust port 24 and a valve 25. Then, the silicon substrates 11 are heated and silane (SiH.sub.4) is introduced through an entrance port 26 and a valve 27. When the substrates are heated to a temperature more than a decomposition temperature of the silane, that is, up to 600.degree.-620.degree. C., the silane is heat-decomposed in a region near the substrates 11 so that a polycrystalline silicon thin film is deposited on the substrate 11. In this case, although the silane is decomposed when a substrate heating temperature becomes 450.degree. C., an amorphous silicon thin film is deposited in the range of 450.degree.-600.degree. C.
However, the grain size of the thus grown polycrystalline silicon is so small as to be about 1 .mu.m or less and defects of a high density exist in grain boundaries. Thus, although it has been merely used as a wiring material of an integrated circuit, it has a problem that when it is used for an active region of a semiconductor element such as a diode and a transistor, that is, when it is used for a raw material for forming a junction, the chracteristics of the element are inferior. The reason why the crystalline grains are small, is that since crystalline nuclei as the origin of crystal growth are formed in various portions of the substrate, the number of crystals is large and when they grow into a size of some largeness, they can not grow further by disturbance of other crystal grains.